A systematic search for spontaneous tumors in inbred strains of mice. Attempt to detect types B or C RNA virus or antigen in spontaneous tumors and other tissues, embryos, and maternal tissues of inbred mice. Define the genetic mechanisms leading to differences in expression of C-type tumor virus(es), including interaction of viral genomes with host genotypes. Develop and expand existing inbred and new recombinant-inbred lines for the purpose of characterizing (immunologically, biochemically, etc.) genetic influences on natural or induced cancer occurrence. Study the association of the viral group-specific antigen or virus with tumor development. Conduct chemical carcinogenesis studies and studies of transplacental carcinogenesis as a rapid system for the evaluation of antiviral vaccines.